1. Field of the Invention
The present invention relates to an improved process for the preparation of diaryl ethers and, more especially, to a process for the preparation of diaryl ethers by reacting an unactivated halobenzene with a phenolate or a naphtholate [hereinafter collectively designated simply as "phenolate"] in the presence of a copper catalyst.
Description of the Prior Art
The aforenoted basic reaction is very well known to the prior art; it is the Ullmann synthesis of ethers, comprising reacting an unactivated halobenzene (i.e., a halobenzene which does not contain, in the position ortho- or para- to the halo group, a group which selectively activates said ortho- and para-positions) with a phenolate, in the presence of a copper compound as a catalyst.
More particularly, British Patent Specification No. 1,052,390 describes the reaction of potassium m-cresolate with bromobenzene, in the presence of activated copper bronze, to obtain 3-phenoxytoluene. Such reaction takes place in bromobenzene at 220.degree. C.-240.degree. C. The main disadvantage of this type of process is that it requires a large amount of the brominated derivative, the latter being used both as a reactant and as the solvent. It too is well known to the art, though, that said brominated derivative is quite expensive. And the high temperature at which the reaction takes place constitutes a further major disadvantage.
Nonetheless, processes do exist which make it possible to utilize the chlorinated derivative. In particular, compare published Japanese Patent Application No. 72/104,672; this publication describes a process for the preparation of meta-phenoxy-toluene, in accordance with which process an alkali metal salt of meta-cresol is reacted with chlorobenzene, in the presence of organic bases, with copper powder or copper compounds being used as a catalyst. The reaction takes place in quinoline at temperatures on the order of 200.degree. C. The great disadvantage of this process, as regards its application on an industrial scale, is no doubt the fact that a solvent such as quinoline involves numerous drawbacks which result from the difficulties encountered in its use and from its high price.
Another published Japanese Patent Application, namely, Application No. 77/035,128, features a process which makes it possible to react chlorobenzene with m-cresolate, in the presence of copper or copper compounds, but in the absence of a solvent.
Utilization of this particular process on an industrial scale, however, presents many serious problems because it must be carried out under pressure and at high temperatures (200.degree.-250.degree. C.).
It will thus be seen that, as regards the preparation of diaryl ethers by reacting an unactivated halobenzene with a phenolate, in the presence of a copper compound, the prior art has yet to provide for a facile and conventional application of the Ullmann reaction on an industrial scale. The requirements which to date have not been met by the prior art can be analyzed in terms of three different points, taken either alone or in any combination. The first concerns the solvent: it would be desirable to be able to use, industrially, solvents which have a low toxicity, as well as good thermal and chemical stability, without at the same time detracting from the economics of the process; in particular, it would be advantageous, in numerous cases, to be able to use one of the reactants as the solvent. The second point concerns the reaction temperature: it is clear that lowering the reaction temperature would constitute a very appreciable advantage both from a technical point of view and also from an economic point of view. In particular, the need to carry out the reaction at the lowest possible temperatures, in the case where the reactants or the products obtained are thermally sensitive to thermal effects, must be emphasized. The third point concerns the halobenzene: as is apparent from the foregoing text, it would be desirable to be able to replace the brominated derivatives by the corresponding chlorinated derivatives under similar reaction conditions.